Slurry Oil/Catalyst Fines Tank Bottoms Recovery & Processing
The problems presented by catalyst attrition from fluid catalytic cracking units (FCCU) have plagued the refining industry since the advent of fluid catalytic cracking in the first half of the 20th century. Over time, FCCU catalyst deteriorates in size. The size-deteriorated catalyst is commonly referred to as “cat' fines”.
In the FCCU process, cracked product stream vapor and some catalyst leave the reactor and enter the main fractionator near its base. On most fluid catalytic cracking units the bottom stream from the fractionator is called heavy cycle oil (HCO) or FCC slurry oil. This FCC slurry oil comprises catalyst fines in a hydrocarbon component, with the hydrocarbon component being comprised of hydrocarbons having aromatics, doubly bonded carbons, carbon-sulfur bonds and carbon-nitrogen bonds. The term slurry oil has arisen as a result of the presence of catalyst particles in the FCC fractionation tower's “bottom product”, i.e. FCC slurry oil. For purposes of the present invention, it is sufficient to know that catalyst particles make their way to the slurry oil product storage tank. Slurry oil is a saleable product of fluid catalytic cracking processing. Once in the storage tank, the catalyst fines settle to the bottom, albeit very slowly. The layering of accumulated FCC catalyst fines that settle to the bottom of the slurry oil storage tank was labeled as a hazardous waste and termed as K-170 in the United States Resource Conservation and Recovery Act (RCRA). The K-170 is also referred to herein as slurry oil cat' fine bottoms or SOCFBs, as noted above. Hence, herein, SOCFBs and K-170 are synonymous.
The existence of catalyst fines in the slurry tank presents a variety of problems to the refiner. The immediate and obvious problem has to do with product contamination. Slurry oil has proven to be an ideal feedstock for carbon black manufacture. Utilization as carbon black feedstock maximizes the value of slurry oil product. However, the presence of catalyst fines above a specified percentage in the slurry oil product results in an “ash content” (i.e., catalyst fines content) in excess of that specification which is acceptable for use of the slurry oil as a carbon black feedstock. Even when the slurry oil product is utilized as a fuel source, a “price penalty” is effectively born as ash content, in the form of the inorganic catalyst fines, increases.
Firms within the specialty chemical industry which service the petroleum refining industry have built proprietary product lines that serve to enhance settling of the catalyst fines in the slurry oil storage tank. In recently or relatively recently cleaned storage tanks this procedure is typically successful in enabling the stored FCC slurry oil product to meet even the rigorous specifications of carbon black manufacturers. As the accumulation of catalyst fines from FCCUs continues in the storage tank, a time comes when no amount of settling enhancement will permit the stored product to “meet specification” of carbon black manufacturers or even fuel products. A second dictate that compels the control of SOCFB accumulation has to do with storage tank inspection criteria. Regulatory authorities require storage tank inspection at specified intervals. The presence of SOCFB's/K-170 interferes with that exercise.
When accumulation of catalyst fines in the FCC slurry oil storage tank becomes intolerable, in terms of meeting product specification or inspection criteria, refinery management schedules a clean-out. The clean-out is typically conducted under one of two typical scenarios. One type of clean-out, referred to as a “partial clean-out” calls for the removal of the catalyst fines without human entry. In this instance, enough of the catalyst fine sediment is removed to make the bottoms manageable once again. Another type of clean-out entails a complete removal of all catalyst fine sediment, subsequent human entry for rigorous clean up and a so-called mop-up, all followed by inspection, repairs and return-to-service.
The low API gravity/high density of the FCC slurry oil, coupled with the entrained catalyst fines, contributes to recovery and handling problems that are reputed to be some of the toughest in the tank cleaning industry. The tank cleaning industry has devised a number of procedures for catalyst fine removal from FCC slurry oil storage tanks. These include the injection of diluent at high pressure either via side ports or from the roof, the cutting of “door sheets” using a water torch and various probe insertion devices. One such insertion device was co-invented by the present inventor and is called the SWEEPBER. It is the subject of U.S. Pat. No. 6,142,160, which serves the purpose of recovering catalyst fines from the bottom of slurry oil storage vessels. A diluent is required to enable ease of handling of the catalyst fine bottoms in all instances known to this inventor. The observed and preferred diluent of choice is Light Cycle Oil or LCO, a side-cut of the FCCU fractionation tower.
The preponderance of catalyst fine projects, observed by the present inventor, are then conducted in a manner described as follows: As removal from the tank is carried out the typical procedure calls for transfer of the slurry oil/catalyst fines/diluent mixture (hereinafter SCDM) to a mobile mix tank, such as that supplied by Baker Tanks Inc., of approximately 22,000 gallons (approximately 500 barrels) capacity. The mix tank has the capability of heating the contents. A heated catalyst fine suspension of pre-specified temperature and concentration is then prepared in the mix tank as feed for centrifuge processing. The desired concentration of cat fines in the SCDM feed to the centrifuge, as relayed by numerous industry professionals, is 15% to 20%.
The heated feed is charged to the centrifuge and processed at a typical rate of 35 gal/min to 42 gal/min. Two streams result from the centrifuge process. One stream is referred to as recovered oil and the second stream is referred to as “filter cake” or “cat fine cake”. The recovered oil, typically having a basic sediment and water (BS&W) content of about 0.5% to 10% (28.5% solids by BS&W has been observed by this inventor), is utilized per refinery management discretion. Typical options include blending the recovered oil into heavy fuel oil products or, depending on BS &W, transfer to slop oil storage inventory for re-processing with the crude oil charge to the refinery crude unit via the desalter.
Pursuant to current U.S. Resource Conservation and Recovery Act guidelines, the filter cake, like the SOCFBs/K-170 itself, is considered a hazardous waste, as noted above. The cost of disposal of this cat fine cake has risen by ten-fold since the mid-1990's and is expected to continue rising. The principal specification that governs the acceptability of filter cake pricing and disposal to a hazardous waste landfill is the “paint filter test”. This test requires the absence of free flowing oil through a standard filter. However, despite the absence of free-flowing oil within the filter cake, a substantial amount of hydrocarbon content remains within the filter cake and, thus, goes unutilized while it remains to impact the environment.
It is not unusual to find that the true hydrocarbon content of post-centrifuged filter cake is greater than 50%. It has been observed that filter cake of high melting point hydrocarbons, such as slurry oil, may contain as much as 83% hydrocarbon. The determination of true hydrocarbon content may be found by conducting a standard ASTM procedure for oil and grease or a true distillation.
There is a currently-used, second method of disposal for filter cake that renders the cake non-hazardous under EPA guidelines. The method is described in U.S. Pat. No. 5,443,717 issued to Robert M. Scalliet; et al. entitled “Recycle of Waste Streams”.
A method for recovering the hydrocarbon component of SOCFB's has been invented by the present inventor and allowed as U.S. Pat. No. 7,244,364 and entitled FCC-CFD Cat′ Fine Desalting: A Method And System For Separating Hydrocarbons And Extracting Catalyst Fines From A Slurry Oil/Catalyst Fines/Diluent Mixture. This FCC-CFD method serves the purpose of recovering the hydrocarbon component of SOCFB's at a recovery rate of 99%+. However, “CFD” does not address the recovery and economic disposition of the catalyst component of SOCFB's. In the instance of the FCC-CFD method, the catalyst component is extracted from the Slurry Oil Cat′ Fines Diluent Mixture (SCDM) into the water layer during the continuous refinery desalting process. The solids contained in the desalter effluent water are classified as a hazardous waste by the US EPA. Alternatively, the present invention serves to both recover the hydrocarbon component of the SCDM with hydrocracker feedstock quality, in terms of low ash content, while “hydrocarbon scrubbing” the catalyst, and rendering the catalyst component as an easily managed dry powder capable of handling by pneumatic systems found in the FCCU make-up catalyst system or in the cement manufacturing industry.
Disposition of Recovered Catalyst
There are two preferred disposition options available as a “closing of the loop” of the catalyst component of SOCFB's. The ideal option is to recharge catalyst, recovered by the LBPSE method, into the FCCU via the make-up catalyst route or slipstreamed into the make-up catalyst system via a proprietary catalyst additive system. Upon recharging of the cat′ fines, a preponderance of the recharged, reclaimed catalyst will attrite via the FCC regenerator scrubber system and thereby be rendered non-hazardous while off-setting makeup catalyst requirements. There is the further benefit of a “metals flush” of mobile vanadium compounds that occurs as vanadium compounds are deposited on the cat′ fines and flushed into the scrubber system.
An ideal situation occurs when an opportunity to cascade this catalyst to an alternate FCCU where there is significant differential, in terms of lesser activity requirement, presents itself.
The second option for the catalyst is as a cement kiln feedstock. The primary chemical constituents of FCC catalyst are the oxides of silica and aluminum; the very same primary components of typical cement kiln feed. Catalyst reclaimed by the LBPSE Method eminently qualifies for this option because of the form in which it is reclaimed, i.e. as a free-flowing powdered material capable of being handled by standard cement kiln pneumatic systems. Catalyst reclaimed by the LBPSE Method contains 2% to as much as 12% hydrocarbon which serves to subsidize the BTU requirements of the cement kiln. The LBPSE method provides that the percentage of hydrocarbon residual remaining in and on the catalyst can be varied to predetermined specification by the number and proportion of extractions.